Currently, nonaqueous electrolyte secondary batteries typified by lithium ion secondary batteries, particularly lithium secondary batteries, are widely mounted on portable terminals, and the like. For these nonaqueous electrolyte secondary batteries, mainly LiCoO2 is used as a positive active material. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g.
Further, as a material of a positive active material for a lithium secondary battery, solid solutions of LiCoO2 and other compounds are known. Li[Co1−2xNixMnx]O2 (0<x≤½), a solid solution having an α-NaFeO2 type crystal structure and formed of three components: LiCoO2, LiNiO2 and LiMnO2, has been published in 2001. LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 that is one example of the aforementioned solid solution has a discharge capacity of 150 to 180 mAh/g, and is also excellent in terms of charge-discharge cycle performance.
In contrast with so called a “LiMeO2-type” active material as described above, so called a “lithium-excess-type” active material is known in which the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is larger than 1, with Li/Me being, for example, 1.25 to 1.6 (see, for example, Patent Document 1 and Patent Document 2). This material can be represented by Li1+αMe1−αO2 (α>0). Here, β=(1+α)/(1−α) when the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is β, and therefore, for example, α=0.2 when Li/Me is 1.5.
Patent Documents 1 and 2 disclose active materials as described above. Further, these Patent Documents describe that a battery capable of attaining a discharge capacity of 200 mAh/g or more can be produced even when a charge method in which a maximum achievable potential of the positive electrode at the time of charge is 4.3 V (vs. Li/Li+) or less or less than 4.4 V (vs. Li/Li+) is employed at the point of use. The battery can be produced by providing, as a production method of a battery including the active material, a production step of performing charging at least up to a region which appears in a positive electrode potential range of more than 4.3 V (vs. Li/Li+) and 4.8 V (vs. Li/Li+) or less and is relatively flat in a potential.
As described above, as distinct from the case of the so-called “LiMeO2 type” positive active material, the so-called “lithium-excess-type” positive active material is characterized in that a large discharge capacity is obtained by performing charge to a relatively high potential exceeding 4.3 V, particularly up to a potential of 4.4 V or more, at least at the first charge.
Further, it is known that by extracting part of Li by acid treatment of the “lithium-excess-type” positive active material, initial efficiency is improved as well as a capacity and cycle performance are also improved (e.g., Patent Documents 3 to 6).
Patent Document 3 describes “A method for producing a positive active material which obtains a positive active material from a lithium-containing oxide comprising a step of treating the lithium-containing oxide with an acid aqueous solution, wherein the lithium-containing oxide includes Li1+x(MnyM1−y)1−xO2 (0<x<0.4, 0<y≤1), the M includes at least one kind of transition metal excluding manganese, and the amount of hydrogen ions in the acid aqueous solution is x mol or more and less than 5× mol with respect to 1 mol of the lithium-containing oxide” (claim 5). Patent Document 3 further describes that an object of the invention is “to provide a positive active material having a high capacity and a method for producing a positive active material which enable excellent load performance and high initial charge-discharge efficiency of a nonaqueous electrolyte secondary battery” (paragraph [0009]).
Patent Document 4 describes “The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the positive active material is represented by a general formula (2): Li2−0.5xMn1−xM1.5xO3 . . . (2), in which Li represents lithium, Mn represents manganese, M represents NiαCoβMnγ (Ni indicates nickel, Co indicates cobalt, Mn indicates manganese, and α, β and γ satisfy 0<α≤0.5, 0≤β≤0.33, and 0<γ≤0.5), and x satisfies a relationship of 0<x<1.00, and the positive active material is obtained by immersing a layered transition metal oxide whose crystal structure belongs to a space group C2/m in an acidic solution” (claim 3). Patent Document 4 further describes that an object of the invention is “to provide a positive active material for a lithium ion secondary battery capable of exerting excellent initial charge-discharge efficiency, and a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery including the positive active material” (paragraph [0008]).
Patent Document 5 describes “A method for producing a positive active material for a lithium ion secondary battery comprising an acid treatment step of bringing an acid solution into contact with an active material represented by a compositional formula: xLi2M1O3·(1−x) LiM2O2 (M1 is one or more kinds of metal elements containing tetravalent manganese as an essential element, M2 is one or more kinds of metal elements, 0<x≤1, and Li may be partially substituted with hydrogen); and a lithium compensation step of bringing a lithium solution containing a lithium compound into contact with the acid-treated active material” (claim 1). Patent Document 5 describes “The method for producing a positive active material for a lithium ion secondary battery according to claim 1, wherein the acid solution is formed of any one of a sulfuric acid aqueous solution, a nitric acid aqueous solution and an aqueous ammonium sulfate solution” (claim 2). Patent Document 5 further describes that an object of the invention is “to provide a method for producing a positive active material for a lithium ion secondary battery capable of suppressing a reduction of a battery capacity due to activation of a positive active material” (paragraph [0011]).
Patent Document 6 describes “A lithium transition metal-based compound powder for a lithium secondary battery positive material, wherein the lithium transition metal-based compound powder is an oxide represented by a general formula (1) and has Li holes and oxygen holes in its crystal structure, and root-mean-square roughness (RMS) of a primary particle surface specified according to JIS B 0601 (2001) is 1.5 nm or less,
xLi2MO3·(1−x) LiNO2 . . . (1) (x is a number satisfying 0<x<1, M is one or more kinds of metal elements with an average oxidation number of 4+, and N is one or more kinds of metal elements with an average oxidation number of 3+)” (claim 1). Patent Document 6 describes “The lithium transition metal-based compound powder for a lithium secondary battery positive material according to claim 1, wherein the lithium transition metal-based compound powder is formed of a compound obtained by performing heating treatment in a solvent with a pH3 of 5 and then performing heat treatment at a temperature of 200° C. or higher and 900° C. or lower for 24 hours or less” (claim 2). Patent Document 6 further describes that an object of the invention is “to provide a positive material for a lithium secondary battery and a positive electrode for a lithium secondary battery which can provide a lithium secondary battery having high initial efficiency and excellent rate performance, and a lithium secondary battery including the positive material and the positive electrode” (paragraph [0010]).